Concepts for wireless energy transmission via laser

Concepts for wireless energy transmission via laser

Leopold Summerer, Oisin Purcell ESA - Advanced Concepts Team

Keplerlaan 1, NL-2201AZ Noordwijk, The Netherlands, Leopold.Summerer@esa.int, +31-71-565-6227

The present paper intends to link several disciplines in an attempt to describe an application of optical systems slightly out of their mainstream applications. Back in the middle age, optics, then the "science of light" has been fundamental in understanding our universe and changing our perspective of our place in it. Optics in form of laser communication and the use quantum encryption are entering the field of space telecommunication and might well reveal to be the single most enabling technology for the introduction of secure and high bandwidth communication. In the present paper a different application of optical links is discussed, using laser links not only as a communications channel but also as a means to transfer energy without wires. Different concepts and applications of wireless power transmission via laser are discussed, including terrestrial and space-based applications.

1. INTRODUCTION

While the science of light and vision has always fascinated and influenced humans, the beginning of modern optics might be traced back to the famous "Book of Optics" by Ibn al-Haytham, which for the first time describes the theory of vision and light as a ray theory, unifying geometrical optics with philosophical physics. His book already described experiments with lenses, mirrors, refraction, and reflection. Modern scientific optics, with the invention of the telescope in the 17th century by Dutch and Italian astronomers and mathematicians revolutionised our way of viewing the universe and the place of Earth and thus ultimately our own place within it.

Optics is already one of the most cross disciplinary disciplines, spanning from physics, chemistry, mathematics, electrical engineering up to architecture, psychology and medicine. This paper intends to describe the application of optics and light in an area where it is traditionally only marginally present: energy transmission.

2. WIRELESS ENERGY TRANSMISSION

The first attempts to transmit energy wirelessly with the purpose of doing so are attributed to N. Tesla at his laboratory in Long Island, New York just 30 years after J. Maxwell had predicted in 1873 the transport of energy trough vacuum via electromagnetic waves, validated in principle 15 years later by H. Hertz?. [1]

Following the invention of the magnetron and the klystron in the 1920 and 1930, the developments during the second world war made microwave beams available to a wider scientific community. The first successful engineering approach to use microwaves for effective energy transmission was done by W. Brown? in the 1960s, by powering among other devices a tethered helicopter. [2]

The first power stations in Earth orbit, taking advantage of the absence of day-night cycles to harvest the energy of the sun were described by the early space pioneers K. Tsiokovski and H. Oberth. Peter Glaser is recognised as the first to combine the visions of these early space pioneers with the practical advances in transmitting energy without wires by W. Brown in his 1968 publication in Science, which contained the first engineering description of a solar power satellite (SPS). [3] It established a vision of a sustainable, practically non-depletable and abundant source of energy to meet world energy demands and triggered the imagination of researchers around the globe.

Since this pioneering article, several small and larger scale studies and experiments have been performed around the world in order to mature the concept of solar power satellites further. For a description of how the concept had evolved since the 1968 publication, it is referred to [4]. While these studies and experiments were generally intensified during times of high carbon fuel prices and received lower attention during times of low oil and gas prices, the idea was never considered mature enough to be put on a larger industrial scale, but the general concept of abundant, virtually CO2 emission free power generated in orbit and transmitted to where needed on

Ibn al-Haytham, 965-1040; also known under the names of

Alhacen or Alhazen Nikola Tesla, 1856-1943 James Maxwell, 1831-1879 ? Heinrich Hertz, 1857-1894

? William Brown, 1916-1999 Konstantin Tsiolkovski, 1957-1935 Hermann Oberth, 1894-1989

Earth has most of the time been considered as too attractive to not pursue further. The most comprehensive study was done during 1979, when the US DoE and NASA made a joint technical analysis (frequently quoted as "SPS reference study") on the options of SPS. [5]

Furthermore, all studied so far essentially concluded that there were no principal technical "showstoppers" to the concept. On the other hand, with each redesign cycle based on new technology, the total mass in orbit, cost and complexity of the entire system decreased substantially, indicating a remaining potential for further improvements. [5] [6] [7] [8]

The most daring concepts was probably proposed by Criswell et al. in 1990: The proposed lunar power stations would be very large installations on the moon, generating energy from solar irradiation on the moon to transmit it to Earth, passing via relay stations and reflectors in Earth orbit. [9] [10]

Following the theoretical works in the 1950s and the first presentation of a functioning laser in 1960, the use of lasers as a means to transmit energy became apparent. [11] [12] At the same time, the first photovoltaic cells were mounted on spacecraft to complement the energy provided by batteries to extend the spacecraft operational life.

The much higher maturity level of microwave devices and the resulting order of magnitude higher overall efficiency has however prevented the concepts of wireless energy transmission by laser to enter into the mainstream SPS concepts for most of the last 30 years. Recent exceptions include the papers of Brandhorst, Steinsiek, Cougnet, Fork, and Luce. [13] [14] [15] [16]

The present paper argues that advances in laser technology and operational as well as engineering advantages of concepts based on laser power transmission provide ground for further interest in this concept and a stronger involvement of the scientific laser community.

This paper concentrates on technologies for longdistance wireless power transmission technologies. Short and medium range wireless power transmission (e.g. via induction or evanescent wave coupling) are not considered. [17]

mosphere; transparency of the atmosphere to the used wavelength;

? possibility for directional emission;

? possibility to convert the energy from the form of its source (solar, electric, heat) to a transmittable form (e.g. microwave, laser, accoustic);

? possibility to convert the transmittable energy form back into a useful form of energy (e.g. electricity, hydrogen).

While this paper concentrates on laser energy transmission, it is useful to compare its performances and parameters with microwave energy transmission, the most widely studied wireless energy transmission technology. In principle, laser energy transmission systems are very similar to energy transmission via microwave technology: the power source (solar, electricity) is converted into an emitter or an emitter array that generates the directional electromagnetic radiation, which is subsequently absorbed in a receiver, which transforms the energy back into a more useful, transportable form, e.g. electricity, heat, hydrogen.

The key difference, the wavelengths used, implies the major other differences between the laser and microwave-based concepts: While most wireless power transmission rely on microwave frequencies of either 2.45 or 5.8 GHz (0.12-0.05 m; both in the industrial, scientific and medical (ISM) frequency band), laser energy transmission takes advantage of the atmospheric transparency window in the visible or near infrared frequency spectrum. (Fig. 1)

3. WIRELESS ENERGY TRANSMISSION TECHNOLOGIES

FIG. 1: Transmission and absorption in Earth atmosphere. (source: NASA)

In general, effective wireless energy transmission concepts need to comply with a range of fundamental constraints:

? possibility to transfer the energy though an at-

For simplicity, only terrestrial applications are considered for the purpose of this paper. The principle remains of course the same of wireless energy transmission between to objects in space and from deep space to planetary surfaces.

The five orders of magnitude frequency difference determine the sizing of the emitters and receiving devices as well as the energy density of the transmission beam according to standard optics principles. Similar to the higher data rate achievable with optical data links (Fig. 2), laser energy transmission allows much higher energy densities, a narrower focus of the beam and smaller emission and receiver diameters.

FIG. 2: Classification of satellite communication systems by beam divergence and data rate. (source: [18])

and used.

The longest distances between emitting and receiving points achieved so far is in the order to hundred kilometers. The largest amount of energy transmitted so far was during an experiment by the US Jet Propulsion Laboratory in 1975, when 30 kW were transmitted from a 26 m diameter parabolic dish to a 1.54 km distant rectenna with 85% efficiency. [19]

The first energy transmission in space between two objects was achieved by N. Kaya et al. in 1983. [20] The first airplane powered by a ground based microwave emitter was launched in Canada in 1987 with the aim to test a wireless power transmission technology to be used for powering high altitude, quasistationary platforms. [21] Similar experiments were performed in Japan in 1993 and 1995, powering from the ground a small airplane and a balloon respectively. [22] [23] The electronic beam steering by phase control of a microwave beam from space to Earth using a pilot signal has been demonstrated in a sounding rocket experiment in 2006 by Kaya et al. . [24]

In a completely different power range and for completely different applications, also the power supply to RFID chips are to be considered an application of wireless power transmission by microwaves. Furthermore, these generally use the same ISM frequency band.

3.1. Wireless power transmission experiments

The principles of wireless power transmission as considered for SPS and other applications has already been demonstrated for both technologies: RF and laser systems.

3.1.1. Microwave-based experiments

Microwave-based experiments have demonstrated so far the possibility to supply power to e.g. helicopters, baloon-based platforms, experimental airplanes, experimental cars, rovers and cell phones. The first experiment was conducted by W. Brown in 1964, when also the first "rectenna"?? and invented

?? A rectenna is a power conversion device from converting microwave to DC. The first rectenna developed by W. Brown was composed of 28 half-wave dipoles terminated in a bridge rectifier using point-contact semiconductor diodes. Later, the point contact semiconductor diodes were replaced by silicon Schottky-barrier diodes which raised the microwaveto-DC conversion efficiency from initially roughly 40% to

3.1.2. Laser-based experiments

While over the years, several laser-based wireless power transmission experiments and applications have been suggested and described, only relatively few actual experiments have been carried out compared to the number and diversity of microwave-based experiments described in the previous section.

Classified experiments involving laser power transmission technology demonstration have been reported to have taken place in the 1980s during the US Strategic Defence Initiative. These seem to have been conducted building on a heritage from the Apollo programme that used ground-based lasers with reflectors on the Moon to measure the Earth-Moon distance. Once of the observatories involved has been the Air Force Maui Optical Station (AMOS) located ontop of mount Haleaki in Hawaii, US. The SDI concepts would use ground based eximer lasers with adaptive optics and a roughly 5 m mirror in GEO and another mirror in a polar orbit at roughly 1000 km altitude. [25] [26] [27]

In 2002 and 2003, Steinsiek and Sch?afer demon-

84%. [2] [19]

strated ground to ground wireless power transmission via laser to a small, otherwise fully independent rover vehicle equipped with photovoltaic cells as a first step towards the use of this technology for powering airships and further in the future lunar surface rovers. [28] The experiment was based on a green, frequency-doubled Nd:YAG laser at only a few Watts. It included the initiation and supply of the rover including a micro-camera as payload as well as the pointing and tracking of the moving rover over a distance up to 280 m by applying active control loops. (Fig. 3)

the phase of a matrix of emitters. Recently, Sch?afer and Kaya have however demonstrated that a similar system is in principle also possible for laser based systems by presenting a new concept for a retrodirective tracking system. [31]

In the proposed concept, the power transmitter utilises a receiver's pilot signal to obtain information about its direction by conjugating the signal's phase inside a nonlinear medium. The emitted power therefore transmitted back to the direction of the receiver by the phase-conjugated signal beam. In this way, power can be concentrated by an array of phase conjugators, which offer the possibility to provide a large aperture in order to increase the intensity at the receiver's photovoltaic panels. The control of the phase and the direction of the readout beams provides control over the interference pattern, its position, and its size, offering new possibilities for the design of spacebased power stations. [31]

3.2. Laser power transmission

FIG. 3: EADS developed, fully laser powered autonomous rover. (source: EADS)

Recently, similar experiments, however focussing less on the beam control and beam steering aspects but rather on the total transmitted power levels have been carried out in the frame of a context related to space elevators, organised and co-funded by NASA. Ground-based lasers have been used to power small PV-covered "climbers" attached to a tether with the objective to achieve maximum climbing speeds. [29] [30]

One of the advantages of microwave power transmission over the use of laser has been the possibility to avoid moving parts in space by using an electronic beam steering system based on the control of

Lasers generate phase-coherent electromagnetic radiation at optical and infrared frequencies from external energy sources by preferentially pumping excited states of a "lasant" to create an inversion in the normal distribution of energy states. Photons of specific frequency emitted by stimulated emission enter and are amplified as standing waves in a resonant optical cavity. The most efficient DC-to-laser converters are solid-state laser diodes commercially employed in fiber optic and free-space laser communication. Alternatively, direct solar-pumping laser generation has a major advantage over conventional solid state or gas lasers, which rely on the use of electrical energy to generate laser oscillation since the generation of electricity in space implies automatically a system level efficiency loss of roughly 60%. To generate a laser beam by direct solar pumping, solar energy needs to be concentrated before being injected into the laser medium. The required concentration ratio is dependent on the size of the laser medium, the energy absorption ratio and the thermal shock parameter (weakness of the material to internal stress caused by a thermal gradient).

3.2.1. Laser selection

In principle, all lasers can be used for transmitting power. Using the general conditions as described in section 3 specifically applied for the selection of lasers, these imply in addition constraints related to the

? efficiency of the laser generation process, and the

? efficiency of the absorption and laser-to-electric conversion processes.

Specifically for direct solar pumped lasers, there are several types of materials suitable as laser medium: From the standpoint of resistance to thermal stress, sapphire seems the optimal material for the laser medium. Since large sapphire crystals are very difficult to produce, most concepts rely on YAG (yttrium aluminum garnet) laser crystals. Concerning the required energy densities, solar energy compression ratios of a few hundred times are required for YAG lasers.

Applications in space or from space to Earth add additional constraints regarding:

? laser generation system mass; ? laser generation temperature requirements

(preference for very high temperature operations in order to allow for a low radiative heat rejection system mass and small size); ? absence of "consumables" and other potential waste products; ? high laser beam quality to avoid the use of lenses and achieve small receiving surfaces; ? control of the phase (arrays of matrices of different laser, possibly used in order to form virtual, large apertures).

FIG. 4: Spectral output of several types of lasers. (source: [32])

pumped lasers use the solar irradiation directly as energy source injected into the laser gain medium.

Under this classification, practically all space based lasers would fall under the category of "solar pumped lasers". Therefore, literature related to space applications usually makes the distinction between standard lasers (in the terrestrial laser power community called indirect solar pumped lasers) and solar pumped lasers (called direct solar pumped lasers in the standard literature on lasers).

3.2.2. Standard indirectly pumped lasers

An analysis of the suitability of different laser types has shown that for the visible frequency range, solid state lasers are in general considered as the most suitable candidates for (space) solar power applications, including diode lasers and diode-pumped thin disk lasers. [33] Especially the later ones have achieved very high power levels of up to kW and overcoming some of the limitations of high power diode lasers, like thermal lensing by reducing the thermal gradients in the material. [34]

In general, these lasers rely e.g. on a laser diode or on materials like Nd:YAG. Currently, the laser diode is the most efficient laser, with an up to 80% plug-in efficiency and an emitted wavelength in the range of 795-850 nm. The most important development effort seems to be made for diodes emitting in the range of 950 nm (pumping of 1.55 ?m fiber laser). For larger scale space applications for wireless power transmission, large area emitting system with thousands of individual diodes could be realised. In this case, the main limitation is the thermal control of such diode panels to maintain optical coherence.

Most of the solid state lasers are based on crystal technology (Nd:YAG, Nd:Y2O3, Ruby, etc). These lasers are optically pumped in the visible range. The Nd:YAG laser (1.064 ?m) is the most widely used; it can be efficiently pumped by laser diodes or solar radiation, emitting visible radiation at 0.532 ?m. The overall system efficiency for the laser diode pumped concept is reported at about 15%. [33]

3.2.3. Direct solar pumped lasers

Scholars on terrestrial solar pumped lasers generally differentiate between two types of "solar pumped lasers": direct and indirect solar pumped versions. In this classification, the "solar pumped" description relates to the sun as origin of the power source, with indirect solar pumped lasers first converting it via e.g. PV panels into electricity which is then used for population inversion inside the gain medium. Direct solar

Direct pumping uses sunlight as the source of the pumping light in order to generate the laser beam. In order to achieve the required power densities for the inversion process, sunlight at 1 a.u. needs to be concentrated from its natural 1387 W/m2 to concentration values between 200 and a few thousands depending on the lasing medium. In order to avoid very large collecting and concentrating surfaces (reflectors

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